1. Field of the Invention
This invention relates to an optical fiber transmission line comprising at least two optical fibers fusion-spliced to each other.
2. Related Background Art
A WDM (Wavelength Division Multiplexing) optical communication system enables long-haul transmission of large amounts of data through propagation of signals for multiple channels of multiplexed wavelengths. In order to enable transmission of large amounts of data over long haul, it is preferred that the optical fiber transmission line, which comprises a transmission medium, have a small absolute value for the accumulated chromatic dispersion in a signal wavelength band. However, in an optical fiber transmission line comprising only a single type of optical fiber, it is difficult to obtain a small absolute value for the accumulated chromatic dispersion in the signal wavelength band. Accordingly, an optical fiber transmission line formed by interconnecting two types of optical fiber having different chromatic dispersion characteristics is generally used.
Specifically, such optical fiber transmission line is formed by connecting a standard single-mode optical fiber having a positive chromatic dispersion and a positive dispersion slope in the signal wavelength band (such as a wavelength band of 1.55 xcexcm) to a dispersion compensator that compensates for the chromatic dispersion in this single-mode optical fiber. This construction results in a small absolute value for the accumulated chromatic dispersion in the entire optical fiber transmission line that includes the single-mode optical fiber and the dispersion compensator. For the dispersion compensator, dispersion compensating optical fiber that has a negative chromatic dispersion and a negative dispersion slope in the signal wavelength band may be used. For example, the dispersion compensator, disclosed in Japanese Patent Application Laid-Open No. 2000-91991, comprises an optical fiber transmission line formed by interconnecting two or more types of dispersion compensating optical fibers in accordance with a predetermined length ratio. Using this dispersion compensator, the absolute value of the accumulated chromatic dispersion over a wide signal wavelength band may be reduced in the entire optical fiber transmission line by compensating for not only chromatic dispersion, but for the dispersion slope as well.
When interconnecting optical fibers having different mode field diameters, such as when connecting single-mode optical fiber with dispersion compensating optical fiber or when connecting two different types of dispersion compensating optical fiber, fusion splicing technology, in which the end surfaces are fusion-spliced after being heated, is generally employed. For example, in the fusion-splicing technology disclosed in Japanese Patent Application Laid-Open No. H3-130705, a first optical fiber having a large core diameter and a small relative refractive index difference of the core with respect to the cladding is fusion-spliced (this process shall hereinafter be referred to as the xe2x80x98fusion-splicingxe2x80x99 process) with a second optical fiber having a small core diameter and a large relative refractive index difference between the core and the clad, the area near the fusion-spliced position is heated, and the dopants in each fiber are diffused (hereinafter referred to as the xe2x80x98dopant diffusion processxe2x80x99). In this way, the difference between the mode field diameter of the first optical fiber and that of the second optical fiber at the fusion-spliced position is kept small, and the connection loss between the first and second optical fibers is reduced.
Therefore, where a single-mode optical fiber (corresponding to the first optical fiber) and a dispersion compensating optical fiber (corresponding to the second optical fiber) are fusion-spliced, the connection loss can be reduced by performing the dopant diffusion process after fusion-splicing. Where the difference between the respective mode field diameters before fusion-splicing (hereinafter the xe2x80x98minimum mode field diameterxe2x80x99) of the first and second optical fibers is relatively large, this process is performed with the intention of reducing the difference between the respective mode field diameters of the first and second optical fibers at the fusion-spliced position. On the other hand, where the difference between the respective minimum mode field diameters of the first and second optical fibers is relatively small, the dopant diffusion process is not performed.
As a result of studying the conventional optical fiber transmission line, the inventors have discovered the matters described below.
During the manufacturing process for the conventional optical fiber transmission line, if the difference between the respective minimum mode field diameters of optical fibers are relatively small, as when dispersion compensating optical fibers are fusion-spliced, the dopant diffusion process is not carried out after fusion-splicing. However, the inventors have discovered that, even where the difference between the respective minimum mode field diameters of the optical fibers to be fusion-spliced to each other is relatively small, if the Ge-concentration in the core region of each fiber is large, and the minimum mode field diameter of each optical fiber is small, the connection loss is large when the first and second optical fibers have been fusion-spliced, and this connection loss has a wavelength-dependency. An optical fiber transmission line having these connection loss characteristics is not desirable for long-haul WDM transmission.
In order to overcome these problems, it is an object of the invention to provide an optical fiber transmission line comprising optical fibers fusion-spliced to each other wherein the difference between the respective minimum mode field diameters of the fibers is small and each fiber has a small mode field diameter and a core region doped with a high concentration of Ge, and wherein the optical fiber transmission line has superior connection loss characteristics.
In this specification, xe2x80x98optical fiber transmission linexe2x80x99 refers to a transmission line comprising first and second optical fibers fusion-spliced to each other and have a mode field diameter difference of 1 xcexcm or less, and includes not only the case in which both the first and second optical fibers are laid in a relay space, but also the case in which the fibers are located in a relay station or the like while one or both of them are wound in a coil configuration and modularized.
The optical fiber according to the present invention comprises a first optical fiber and a second optical fiber fusion-spliced to each other. The first optical fiber has a core region doped with a high concentration (10 mol % or more) of Ge, and a small mode field diameter in which the minimum value at a wavelength of 1550 nm is 7 xcexcm or less. The second optical fiber as well has a core region doped with a high concentration (10 mol % or more) of Ge, and a small mode field diameter in which the minimum value at the wavelength of 1550 nm is 7 xcexcm or less. Accordingly, the difference between the respective minimum mode field diameters of the first and second optical fibers is 1 xcexcm or less.
In particular, in this optical fiber transmission line, the mode field diameter of the first optical fiber, at a position separated by at least 2 mm from the fusion-spliced position between the first and second optical fibers, is enlarged so as to reach a value of 110% of the minimum mode field diameter of the first optical fiber. Similarly, the mode field diameter of the second optical fiber, at a position separated by at least 2 mm from the fusion-spliced position between the first and second optical fibers, is enlarged so as to reach a value of 110% of the minimum mode field diameter of the second optical fiber.
As described above, a conventional optical fiber transmission line, which is formed by fusion-splicing first and second optical fibers having a small difference between their respective mode field diameters, has a large connection loss, and this connection loss is highly wavelength-dependent. However, because in the optical fiber transmission line according to the present invention, the respective mode field diameters of the fibers are intentionally enlarged in the respective range of each fiber up to 2 mm from the fusion-spliced position between the first and second optical fibers, both the connection loss and the wavelength-dependency regarding the connection loss are reduced. It is preferred that the enlarged amount of mode field diameter in each of the first and second optical fibers at the fusion-spliced position therebetween be 0.5 xcexcm or more. In this case, the connection loss and the wavelength-dependency thereof are further reduced. It is furthermore preferred, in this optical fiber transmission line, that the end portions of the first and second optical fibers up to 2 mm from the fusion-spliced position therebetween be heated by using a heat source such as a micro-torch or a heater after fusion-splicing the first and second optical fibers, such that the mode field diameters of the first and second optical fibers will be enlarged.
The mode field diameter referred to in this specification means as the Peterman II mode field diameter at the wavelength of 1550 nm. The minimum mode field diameter corresponds to the Peterman II mode field diameter of the optical fiber before fusion-splicing, and substantially corresponds to the Peterman II mode field diameter in the region other than the region in which the mode field diameter is enlarged after fusion-splicing (i.e., the mode field diameter of which no fluctuation occurs before or after dopant diffusion processing). Furthermore, the rate of enlarge in the mode field diameter at a position at least 2 mm from the fusion-spliced position between the first and second optical fibers (i.e., the fusion-spliced end surface of each optical fiber) is set to be 110% or more of the minimum mode field diameter of each optical fiber. Because the mode field diameters at every parts of the optical fiber do not necessarily match, and there is a certain degree of variation when the optical fiber is manufactured, this setting is performed in order to enable the region in which the mode field diameter fluctuates naturally due to this variation during manufacturing to be distinguished from the region in which the mode field diameter is enlarged intentionally.
In the optical fiber transmission line according to the present invention, each of the first and second optical fibers comprises a core region extending along a predetermined axis and having a first refractive index, a first cladding region provided on the outer periphery of the core region and having a second refractive index lower than the first refractive index, and a second cladding region provided on the outer periphery of the first cladding region and having a third refractive index higher than the second refractive index. Furthermore, in the optical fiber transmission line, it is preferred that each of the first and second optical fibers comprises a core region extending along a predetermined axis and having a first refractive index, a first cladding region provided on the outer periphery of the core region and having a second refractive index lower than the first refractive index, a second cladding region provided on the outer periphery of the first cladding region and having a third refractive index higher than the second refractive index, and a third cladding region provided on the outer periphery of the second cladding region and having a fourth refractive index lower than the third refractive index. Where the first and second optical fibers have the structure described above, an optical fiber transmission line in which the mode field diameters are not enlarged near the fusion-spliced position, as in the conventional art, has a markedly high connection loss, but in this optical fiber transmission line, both the connection loss and the wavelength-dependency regarding the connection loss are reduced.
It is preferred that each of the first and second optical fibers has, as characteristics at the wavelength of 1550 nm, a chromatic dispersion of xe2x88x92200 ps/nm/km to xe2x88x9280 ps/nm/km and a negative dispersion slope. This type of optical fiber is appropriate as a dispersion compensating optical fiber placed in a relay space. It is even more preferred that each of the first and second optical fibers have, as characteristics at the wavelength of 1550 nm, a chromatic dispersion of xe2x88x9260 ps/nm/km to xe2x88x925 ps/nm/km and a negative dispersion slope. This type of optical fiber is appropriate as a dispersion compensating optical fiber placed in a relay space after it is modularized by being wound in a coil configuration. In either case, the optical fiber transmission line according to the present invention offers reduced connection loss and reduced wavelength-dependency regarding this connection loss.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying views which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.